The present invention relates to a multi-beam scanning method wherein multiple beams are deflected by a rotary polygon mirror and then, are converged by an image focusing optical system to spots, whereby the surface of an image carrier is scanned by plural scanning lines simultaneously, and to a multi-beam scanning control apparatus employing the multi-beam scanning method.
For attaining the demand for high speed recording, there has been known a multi-beam scanning control apparatus (see TOKKAIHEI 2-188713) wherein multiple beams emitted from plural semiconductor lasers are deflected by a rotary polygon mirror or the like, and then are converged by an image focusing optical system into spots, and the surface of an image carrier is scanned by plural lines simultaneously, thus two-dimensional images are formed while the image carrier is rotated in the sub-scanning direction that is perpendicular to the primary scanning direction. In the multi-beam scanning control apparatus, it is necessary to correct a displacement quantity based on a standard beam and thereby to conduct synchronization control, for preventing optical axis displacement in the primary scanning direction and the sub-scanning direction for multiple beams. In the multi-beam scanning control apparatus employing such synchronization control, incidence position in the primary scanning direction on the image carrier of multiple beams is detected by an index sensor provided in the vicinity of the image carrier surface, and whereby horizontal synchronization signals are obtained. On the other hand, for stabilizing potential of a latent image formed on the image carrier, there is a technique that a semiconductor laser beam light quantity is stabilized based on signals obtained when a semiconductor laser beam light quantity is subjected to sample-and-hold for each scanning line, and the technique is abbreviated as line APC hereinafter.
FIG. 13 is a time chart showing operations of a non-semiconductor laser control circuit. FIG. 13(a) is a time chart showing the state of sample-and-hold, in which a high level represents the sample state and a low level represents the hold state. FIG. 13(b) is a time chart showing the state of input of image data, FIG. 13(c) is a time chart showing an electric current for driving a semiconductor laser, and FIG. 13(d) is a time chart showing output of position detection for the standard beam.
Time charts shown in FIG. 13 indicate that a line APC scanning control circuit in a conventional individual beam has obtained horizontal synchronization signals with detection signals from an index sensor at the end of sample processing for the so-called line APC.
However, when attempting to conduct multi-beam scanning control with the control timing of line APC in a single beam like that stated above, it has been difficult to detect a laser beam which represents a standard among plural laser beams entering an index sensor because a beam position detection is conducted without controlling properly a quantity of light of multiple beams entering the index sensor, resulting in problems. This will be explained in detail as follows.
FIG. 14 is a conceptual diagram showing the state of scanning on an index sensor by multiple beams.
If there is no optical axis displacement in the primary scanning direction for each laser beam, when multiple beams enter a light-intercepting portion of the index sensor simultaneously as shown in FIG. 14(a), a quantity of light intercepted by the index sensor is larger than that for a single beam as shown in FIG. 14(d). Such phenomenon can be a cause for malfunction when a displacement amount is detected and corrected based on the standard beam and thereby synchronization control is performed.
If there is an optical axis displacement in the primary scanning direction for each laser beam, the standard laser beam does not always enter the light-intercepting portion first as shown in FIG. 14(b). The detecting circuit judges inadvertently that the signal inputted first is a standard signal, and fails accordingly to detect the standard laser beam accurately as shown in FIG. 14(c), resulting in an impossibility of detection of an accurate displacement amount and of correction therefor.